1. Field of the Invention
The present invention relates to the localized surface metallization of pieces of composite material and more particularly of pieces, particularly adapted to be mounted on aircraft, constituted of a composite material covered with a layer of an electrically conductive material to protect against lightning or against too strong electric charges for example, and, of a protective coating of the surface of said metallic layer, formed by a layer of paint, of anodization, or the like.
2. Description of the Related Art
After producing by moulding, these pieces are then pierced in different locations to be able to be fixed to the place to which they are destined. Certain of the pierced points will be used as contact points for grounding.
As the pieces are painted or coated with an electrically insulating corresponding protective layer, it is necessary to free the periphery of the selected holes to expose the electrically conductive layer, this operating being called metallization. This has been done until now by a long, difficult and costly process which consists in the following steps:
moulding the composite piece with the layer of conductive material at the surface (generally a metallic sheet of aluminum or bronze for example).
piercing/shaping of holes with the aid of a computer-controlled machine.
manually masking, with the aid of small sticky dots, the holes before undergoing metallization.
painting the piece, the term xe2x80x9cpaintingxe2x80x9d here meaning, as well as in the description which follows, any protective coating covering the conductive layer of the piece.
unmasking the holes and manually removing the dots.
manually clearing the holes that were not masked (non-metallized holes) to open the holes filled with paint.
manually finishing of the holes to be metallized. This last operation is delicate, long, difficult and adapted to cause defective metallization.
Thus, despite the presence of the dots, the paint leaks even under the dots, thus forming a layer of paint at the periphery of the holes to be metallized. It is thus necessary to scalp this periphery to remove any trace of paint, as well as any portion of the resin of the composite material adapted to cover the conductive layer, generally a metallic cloth which is embedded at least partially in said resin.
This exposure of the conductive layer is carried out by a worker who introduces into the bore the free end of a metallizing brush driven in rotation and strips the peripheral region of the opening of the bore. Then, visually, the worker verifies the work that has been carried out. If paint remains, he does it again. Generally, he repeats a large number of times for a same piece, with the risk of going too far, which is to say also removing pieces of the conductive layer. The integrity of this latter is thus lessened, which risks degrading the quality of the electrical contact later established for the purpose of grounding.
The risk of damaging the integrity of the conductive layer is not small, in contrast to the layer of paint which itself is very thin, the piece being moreover most often contoured, such that in the same region there can be found, in one place, some paint, and a little ways away, a portion of removed conductive layer.
Such a process thus gives rise to long production cycles and variable quality, in addition to the cost arising from the skilled work of the metallization.
So as to overcome these drawbacks, various paths have been followed, such as improvement of the metallizing brushes, the use of various assemblies of metallizing brushes, or else recourse to sanding or abrasives.
None of these techniques is a completely satisfactory solution to the problems of metallization, particularly because of the requirement for the use of visual examination by the worker and the risk, present no matter what the means used, of going too far in removing the paint.
The present invention has precisely the aim of solving these problems by providing a metallization process adapted to carry out automatically optimum metallization, which is to say with controlled exposure, if desired total exposure of the conductive layer, without impairing the physical integrity of this latter.
To this end, the invention has for an object a process for localized surface metallization of pieces of composite material, of the type covered with a surface layer of an electrically conductive material, itself coated with an electrically insulating layer, such as paint or the like, characterized in that after moulding of said piece of composite material with said conductive surface layer, all the surface of said paint or the like is coated on the conductive layer side of the piece, then, in line with each region of said surface dedicated to a hole to be metallized, there is carried out a scraping off or leveling until the conductive layer is exposed, with the help of a metal edge that is pressed resiliently and moved against the surface of the piece, the end of the scraping off being controlled consecutively upon reaching or exceeding a predetermined threshold of electrical conductivity between said conductive layer and said metallic edge and, finally, the various holes of the piece are pierced and shaped.
Such a process can be practiced by a computer-controlled machine carrying out all the steps of said process, automatically, and on pieces that can have complicated shapes, particularly irregular surfaces.
The invention also has for its object a metallizing tool particularly suited for carrying out scraping off or leveling according to the above process.
This tool, more particularly adapted to be mounted on a milling head of a multi-axle digitally-controlled machine, is constituted by a rotating spindle provided at its end with a transverse arm carrying at one end a counterweight and at its other end a tool-carrying arm articulated on said transverse arm about an axis orthogonal to the axis of the spindle, said tool-carrying arm being turned toward said axis of the spindle and carrying a plate of carbide or the like, positioned so as to form a negative cutting angle and to define a scraping edge whose one end is in line with the axis of the spindle, return means being provided between the transverse arm and the tool-carrying arm resiliently to press the plate or the like against the surface to be scraped off, said plate being electrically connected to a contact detector which itself is connected, on the one hand, to the conductive layer of the piece to be metallized by a removable connection and, on the other hand, to the digital control of the multi-axle machine, said detector comprising a current generator and controlling the stopping of said rotatable spindle when it reaches or exceeds a predetermined threshold of intensity of current flowing between the conductive layer and the plate.
Said threshold of current intensity is of course variable and depends particularly on the quality of metallization that is sought.
When the carbide plate touches the conductive material of the piece, the current passes and the intensity of this current is a good indicator of the degree of stripping of the conductive layer.
Preferably, the carbide plate is mounted on the arm oscillatingly, by an axle about which the plate can pivot so as continuously to match the surface to be scraped off, particularly when it is convex or concave.
According to one embodiment, the electrical connection between the plate and the contact detector is effected by an electrical conductor connecting the plate to a collector arranged on the rotating spindle and in contact with a brush itself connected to said detector.
Preferably, such a connection can be divided for detecting anomalies in this connection, for safety purposes.